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To: cs551@merlot.usc.edu
Subject: Re: reagarding select()
Date: Tue, 28 Oct 2008 14:25:30 -0700
From: Bill Cheng <william@bourbon.usc.edu>

Someone wrote:

  > The syntax for select is
  > int select( int nfds,fd_set *read,fd_set *write, fd_set *error,Struct
  > timeval *timeout))
  > 
  > Now , i have multiple  readind threads calling select().Each select() scans
  > socket descriptors from 0 to nfds-1.
  > Will a reading thread 's(with sock fd =5) select() unblock if another
  > reading thread (with sock  fd = 3) receives data .
  >    Beacause the former's select will scan from sock fds 0-4.

You need to use FD_SET() to specify which file descriptors
are you interested in for a specific select() call.  For
example, if your thread is only interested in reading fd=5,
then you should do:

    fd_set fds;
    struct timeval timeout;

    FD_ZERO(&fds); /* it's imperative that you initialize all variables */
    FD_SET(fd, &fds);
    .... /* set timeout */
    return_code = select(fd+1, &fds, NULL, NULL, &timeout);
    
Although it will *scan* from file descriptors 0 through 5, it
will skip the ones that you did not call FD_SET() on.
--
Bill Cheng // bill.cheng@usc.edu <URL:http://merlot.usc.edu/william/usc/>